Anybody who lives with pets knows they can have mood swings. I have the cat scratches to prove it. But can animals actually lose their minds like we do? In 2012 alone an estimated 2.8 million dog owners gave their dog some sort of anxiety medication. Tonight the author of a provocative new book and the goat who she says proves her thesis. Reporter: This goat named Mr. G. Is depressed. For years he lived alongside a female donkey named jellybean. At the home of an animal hoarder. But when the animals were rescued and taken to separate shelters, Mr. G. Refused to go outside or eat for six days. In a moment, what makes Mr. G. Perk up for the first time. But first, consider this -- Humans and other animals can lose their minds in ways that are really similar. Reporter: Historian of science, Dr. Laurel braidman in a new book says animals and humans are, quote, astonishingly similar when it comes to their feelings, suffering from the same psychological conditions, like depression, anxiety, even OCD. When we diagnose animals with the same sorts of mental illnesses that we have, are we taking a leap? How do we really know what's going on with these animals? They can't talk to us. We don't really know what's going on with these animals. We diagnose people without asking them questions verbally all the time. Reporter: She points out that mentally ill animals are often treated in the same way as humans with the same medication, and often like us, with plain old love, even if it's not from their own species. I've seen this in my own travels. How are you doing? What's going on? Reporter: These orphan babies in the Congo who thrive with surrogate human moms. Which brings us back to Mr. G. Look what happens when he hears his long-time companion, jellybean arriving at the shelter. He can barely believe his eyes it seems. There's a little kiss, and then Mr. G. Eats for the first time in six days. An interspecies love story that opens a window into how all of us can heal. Who knew the story of a goat and a donkey could be so moving. Dr. Laurel braidman's new book called "Animal madness" is in stores right now.

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